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family demonstrated that this system involves thioredoxin, not GSH (Comtois
et al.
2003
; Iqbal et al.
2006
; Miao et al.
2006
). This suggests that GSH does not
contribute to the scavenging of ROS via GPX. The seven members of the GPX
family in
Arabidopsis
localise to different subcellular sites (Millar et al.
2003
).
The overexpression of GPX enhances the tolerance of transgenic plants to abi-
otic stress. Mutation of
AtGPX3
impairs ABA and drought stress responses, and
AtGPX3 functions to control water loss from the stomata by its interaction with
ABI2 in the ABA signalling pathway (Miao et al.
2006
). Recent results indicated
that the deficiency of AtGPX8 accelerated the progression of oxidative stress in
AtGPX8
knockout plants (Gaber
2013
). Plant GST enzymes (EC 2.5.1.18) cata-
lyse the conjugation of electrophilic xenobiotic substrates with GSH. GST can
also reduce peroxides by using GSH. The ability of GST enzymes to remove cyto-
toxic or genotoxic compounds can protect DNA, RNA and proteins from damage
(Noctor et al.
2002
). Plant
GST
gene families are large, with 25 members in soya-
bean, 42 in maize and 54 in
Arabidopsis
(Dixon et al.
2002
; Sappl et al.
2004
).
GST
overexpression can also enhance plant tolerance to various abiotic stresses.
For example, tobacco seedlings that overexpress
GST
and
GPX
showed enhanced
growth under thermal- or salt-stress conditions (Roxas et al.
2000
).
Numerous reports have demonstrated that ABA affects the abundances of scav-
enging enzymes at the transcriptional and post-transcriptional levels. Treatment of
Syzygium cumini
with ABA caused a drastic decrease in CAT activity but increase
in APX transient and the increase in SOD sustained. H
2
O
2
treatment was also very
effective in increasing APX, CAT and SOD enzyme activities (Choudhary et al.
2012
). On this issue of transcription regulation, maize calcium-dependent protein
kinase 11 (ZmCPK11) is involved in ABA-induced up-regulation of the levels of
expression and activities of SOD and APX, and in the production of H
2
O
2
(Ding
et al.
2013
). As another example of OsDMI3 encodes a rice Ca
2
+
/CaM-dependent
protein kinase, which is required for ABA-induced increases in the expression and
activities of SOD and CAT and the production of H
2
O
2
(Shi et al.
2012
), the ABA-
activated mitogen-activated protein kinase (MAPK) OsMPK1 is also involved in
this process (Shi et al.
2014
).
Increased drought and salt tolerance in transgenic plants is associated with
ABA-induced production of H
2
O
2
via NADPH oxidase and NO via NOS-like
enzymes, which sequentially induce the transcription/translation and activi-
ties of SOD, CAT, APX and glutathione reductase (GR) (Zhang et al.
2009a
,
b
).
Photosynthetic redox signals and ABA signals integrate antagonistically at a more
distally located promoter region, called a redox box. There, the APETALA-2-type
transcription factor RAP2.4a, induces
2CPA
transcription upon moderate oxidative
stress (Shaikhali et al.
2008
). However, conflicting results have indicated that ABA
significantly reduced the activities of SOD, APX, POD and CAT, as well as the
levels of GSH and ASA, during adventitious rooting (Li et al.
2014
).
Non-enzymatic low-molecular-weight metabolites
: The non-enzymatic low-
molecular-weight metabolites that protect against oxidative stress are ascorbic acid
(ASH, vitamin C), GSH,
ʱ
-tocopherols (vitamin E), carotenoids, flavonoids and
proline (Mittler et al.
2004
; Chen and Dickman
2005
).
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